Vehicle manufactures are required to provide seatbelt restraint systems in almost all road vehicles today. Typically, these restraint systems include a retractor, a seatbelt webbing, a latch plate and a seatbelt buckle. When the seatbelt system is not in use, the seatbelt webbing is retracted into the retractor and wound about a spindle disposed in the retractor. When in use, the seatbelt webbing is protracted from the retractor, wrapped around the occupant and secured by the latch plate to the seatbelt buckle.
In an effort to improve the safety of an occupant during a vehicle crash, retractors having torsion bars have been developed to limit the forces applied by the seatbelt on the vehicle occupant. The torsion bar is fixed to a spindle on a first end and to a tread head on a second end. As the seatbelt webbing is loaded by a vehicle occupant during a crash, the torsion bar twists allowing the spindle to rotate and pay out additional webbing to reduce the peak forces on the occupant. Depending on the occupant's size and the severity of the crash, different amounts of webbing pay out and a corresponding level of twisting of the torsion bar is appropriate.
To achieve different load limiting characteristics and degrees of webbing pay out during a vehicle collision, retractors have been developed having multiple torsion bar segments which may be activated independently depending on the size of the occupant and the severity of the collision. While these new and improved multi-level load limiting retractors achieve their intended purpose, many issues still exist. For example, manufacturing of torsion bars that provide multiple levels of load limiting are much more complex and, consequently, more expensive to manufacture than retractors having a single load limiting level torsion bar. Therefore, a need exists for a method of constructing multi-level load limiting retractors in a feasible and cost effective manner.